Im going through this right now. Well, mostly building my own harness.
The advise I can give you is, search. If you need to ask what sand paper you need to use, you should probably leave it to a pro. Not to discourage you from doing it, but you know.

As far as building my own harness, I just got a bunch of oem manuals that are free online. I've never done one before, so I just cut all the clips off, extended the wires and soldered everything to the ecu main clips. If you have an odb2 its should be ten times easier. Im doing it on a crx with obd1 harness and also cutting everything I don't need from the engine harness and main cabin harness.
headlights and corners are super easy, like five wires oer side including dual horns.
If you can't find the manuals I can email them to you or post them on here?
Or? Just give RC a call and get it done right the first time.